1. Field of the Invention
This invention relates to a technique for converting a number of bits of digitally modulated channel bits and demodulating the same. More particularly, it relates to a technique for converting a number of bits of channel bits and demodulating the same in such a manner that signals from an optical disk may be commonly processed whether it is the DVD (Digital Video Disk) or the CD (Compact Disk).
2. Description of Related Art
As information distribution media employing the optical digital recording, there are CD and DVD. Different types of CDS include CD-ROM for read-only, CD-DA for audio, CD-WO for write-once and CD-R for recordable, all of which constitute the CD family by virtue of the uniformly defined recording/regenerating formats. Similarly, different types of DVDs constitute the DVD family.
Data bits of information to be recorded on CD or DVD are modulated into channel bits by its unique modulation scheme that complies with its unique RLL constraints, whereas a respective demodulating reference table is referred to for demodulating the original data bits during its playback. With regard to modulation of data bits, a notation xe2x80x9cRLL (d, k)xe2x80x9d represents that the number of successive xe2x80x98ZEROSxe2x80x99 occurring between xe2x80x98ONESxe2x80x99 in channel bits may be within the range of at least xe2x80x9cdxe2x80x9d (minimum run-length) and at most xe2x80x9ckxe2x80x9d (maximum run-length). Further, NRZ (NonReturn-to-Zero) or NRZI (NonReturn-to-Zero Inverted) recording method is employed to record data in such a way that a polarity of a bit xe2x80x98ONExe2x80x99 is inverted at its leading edge or center portion only to represent presence/absence of a pit on an optical disk, while such an inversion is not done for a bit xe2x80x98ZEROxe2x80x99. As a result, a modulation scheme that complies with its associated RLL constraints limits the minimum and maximum numbers of successive absences of pits on an optical disk. In so doing, during regeneration of information, it is possible to extract bit clock components for controlling a servo subsystem in a stable manner, and yet it is possible to obtain better regeneration characteristics that are free from wave interference due to a diffraction limit.
The EFM (Eight-to-Fourteen Modulation) modulation scheme has been employed for CD, whereas the 8/16 (alternatively called xe2x80x9cEFMPlusxe2x80x9d) modulation scheme has been employed for DVD. One symbol of data bits is 8 bits for both of EFM and 8/16 modulation schemes, whereas one symbol of channel bits is 14 bits long for EFM modulation scheme and 16 bits long for 8/16 modulation scheme respectively. Note here that the term xe2x80x9csymbolxe2x80x9d means a word unit for error correction. Since both of CD and DVD employ different modulation schemes respectively, when one wishes to play back both of CD and DVD by means of a single playback apparatus, it has been required to provide signal processing circuitry (which is responsible for a sequence of operations, from detection of a sync bit pattern in a channel bit stream that enters into the demodulation subsystem up to output of regenerated data bits) for each of the modulation schemes respectively, in addition to provision of individual demodulating reference tables. FIG. 1 is a schematic block diagram showing a conventional optical disk playback apparatus that is capable of playing back both of CD and DVD.
As seen in FIG. 1, information on a medium 10 (CD or DVD) read by an optical head 11 is sent to a DVD data processor 16 and a CD-DA processor 17 via an RF preamplifier 12 as RF analog signals of a channel bit stream. However, data processor 16 or 17 operates to play back its associated medium alone. That is, data processor 16 including an 8/16 demodulator is only used during a DVD playback, whereas data processor 17 including an EFM demodulator is only used during a CD playback. After performing predetermined processing, its resultant data is sent to an interface 18 or a CD audio amplifier 19. Both of the 8/16 and EFM demodulators are provided with memories for storing demodulating reference tables, which are uniquely associated therewith respectively.
When channel bits of 16 bits long are treated as directly indicating an address of a ROM in the 8/16 demodulator for storing its demodulating reference table, the memory requires a space of 65,536 addresses. On the other hand, the demodulating reference table in the EFM demodulator requires a space of 16,384 addresses in its ROM since its associated channel bits are 14 bits long.
In said playback apparatus, both of CD and DVD are not concurrently played back. This means that while one of the media is being played back, a data processor associated with the other media is not being operated, which leads to a reduction in availability of its signal processing circuitry. Also, each of the demodulating reference tables separately requires a certain amount of memory capacity respectively, which leads to heavy consumption of memory resources.
It is, therefore, an object of this invention to provide a technique for converting numbers of bits of channel bits modulated by first and second modulation schemes into lesser numbers of bits in such a manner that regenerated signals may be commonly processed by a single processing path.
It is another object of this invention to provide a demodulating reference table, which is formed by consolidating respective reference tables used for demodulating channel bits modulated by first and second modulation schemes, and which is storable in a small capacity memory.
It is another object of this invention to provide a technique for demodulating channel bits that makes use of said converting technique and said demodulating reference table. It is another object of this invention to provide an optical disk playback apparatus characterized by demodulating circuitry for regenerated signals.
Numbers of bits of channel bits to/from CD and DVD are determined by RLL constraints so as to improve recording/playback characteristics of a medium. Note that among those patterns representable by a predetermined number of channel bits, some patterns may not actually exist under the RLL constraints. When channel bits are picked up from an optical disk and introduced into a regeneration subsystem, a number of bits for representing respective patterns of channel bits may be reduced by associating the patterns with other patterns represented by other bits having a lesser number of bits than the channel bits. A fundamental principle of this invention resides in generating signals for use in a playback by converting a number of bits of channel bits modulated by a first or second modulation scheme. Also, another principle of this invention resides in creating a consolidated reference table for use in demodulating patterns of first or second channel bits, thereby enabling to store it into a memory of a smaller address space.
In one aspect of this invention, there is provided a method and an apparatus for converting first or second channel bits, modulated by a first or second modulation scheme, to first or second output bits respectively. While the output bits may be used as address bits for referring to an address of a memory that stores a demodulating reference table, they may be used in a signal processing subsystem as well for processing signals detected from an optical disk. In the bit conversion of this invention, respective patterns actually represented by the channel bits before-conversion are associated with those patterns that are representable by the output bits after-conversion, which have a lesser number of bits than the channel bits. Respective output bits are generated in such a way that they represent patterns, which correspond to patterns represented by the channel bits other than those patterns being unable to exist under RLL constraints. Further, patterns of the second output bits are generated in such a way that they are assigned to discontinuous areas of patterns represented by the first output bits. In so doing, it becomes possible to reduce a number of bits of respective output bits than that of respective channel bits, which in turn enables to reduce a bus width for lessening a burden to the subsequent signal processing subsystem, and yet to unifies the signal processing subsystem. Further, the first and second output bits may be used for addressing the common reference table of a small address space.
In another aspect of this invention, said first modulation scheme is 8/16 modulation scheme, whereas said second modulation scheme is EFM modulation scheme. Both of these modulation schemes are being employed for optical disks of the DVD and CD families respectively, and they may be used in the signal processing subsystem for regenerating signals from both types of optical disks.
In yet another aspect of this invention, there is provided an apparatus for converting channel bits to output bits. While means for identifying channel bits, means for generating first output bits and means for generating second output bits may be implemented by one or more stored program processors, it is preferable to implement these means by one or more wired logic circuitry each comprising a combination of logic elements for causing bit operations to be directly performed. This is because bit processing for demodulation has to be continuously performed at high-speed.
In another aspect of this invention, there is provided a method of creating a reference table for demodulating patterns of first and second channel bits, modulated by first and second modulation schemes respectively. First and second address patterns correspond to patterns, which are representable by all of respective channel bits other than those patterns being unable to actually exist within the said respective channel bits. Accordingly, a number of bits for representing respective address patterns may be smaller than that of respective channel bits. Since the second address patterns are assigned to discontinuous areas of the first address patterns, it is possible to embed all of the second address patterns within the range of an address space that is defined by a number of bits for representing the first address patterns. Both of the address patterns are associated with patterns of respective data bits through corresponding patterns of respective channel bits.
In yet another aspect of this invention, there is provided a method and an apparatus for demodulating patterns of first and second channel bits, modulated by first and second modulation schemes respectively. Respective channel bits are converted to respective output bits, after identifying the said channel bits modulated by its associated modulation scheme. A demodulating reference table is created by associating the first and second address patterns with patterns of the data bits. Since the second address patterns are assigned to discontinuous areas of the first address patterns, there is no need to enlarge an address space even where an address pattern is referred to by bits. Referring to respective address patterns using patterns that are represented by respective output bits, it is possible to obtain patterns of the data bits via the output bits and the address patterns. In so doing, it is possible to consolidate respective reference tables associated with the first and second modulation schemes into a common reference table, thereby reducing an address space required for accommodating the same.
In another aspect of this invention, there is provided an optical disk playback apparatus having a demodulator of the type as set forth above. This demodulator employs a data bus that is narrower than a data bus required for channel bits, and it is capable of demodulating signals from at least two different optical disks by making use of a single reference table.